The present invention relates to a method of determining at least one energy property of a gaseous fuel mixture by measuring physical properties of the gaseous mixture, determining the composition of a gas equivalent to said gaseous mixture and deducing the energy properties from said composition.
The energy properties of a gas, such as the calorific value, the Wobbe index, the stoichiometric air/fuel ratio or the methane index, are of great industrial interest. In fact, a variation in the composition of a gas (for example natural gas) due to there being a large number of sources of supply (Algerian gas, Norwegian gas, Russian gas, etc.) can cause serious damage in stationary gas engines. These engines are generally used for the simultaneous production of heat and electricity (cogeneration). Furthermore, the efficient use of gaseous fuels in internal combustion engines depends mainly on their ignition properties and combustion properties.
The energy property which makes it possible to follow the variations in the quality of natural gas in terms of the antiknock value is the methane index.
Fuel gases, which are of relative importance in terms of the very different energy properties, also have diverse origins: wood gas, coal gas, natural gas, etc.
When biogas is used to drive internal combustion engines, a variation in the composition of the gas can have serious effects on the performance characteristics of the engine. For example, there can be power fluctuations due to the variation in net calorific value (the NCV can vary from 10 to 25 MJ/m3). Thus, for optimal operation of the engine, it becomes essential to measure the NCV of the biogas.
The Wobbe index is another important energy property of gaseous fuels (it can vary from 10 to 30 MJ/m3). It is an important criterion of the interchangeability of gases in engines. A variation in composition does not cause an appreciable change in the air factor or the combustion rate if the Wobbe index remains almost constant. This index can be deduced from calculation of the calorific value by means of the following relationship:
                    W        =                  NCV                      d                                              (        1        )            
where NCV is the net calorific value of the gas and d is the density of the gas.
The quality of gaseous fuels can be measured by numerous techniques, among which there may be mentioned the techniques of measuring the calorific value and the technique of measuring the methane index.
a) Methods of Measuring the Calorific Value
Given the composition of a gaseous mixture, it is easy to calculate its calorific value by using the specific value of each of the constituents of the gas in question.
Direct determination of the calorific value can be effected by means of hand-operated calorimeters and automatic calorimeters, for example the bomb calorimeter, Junkers calorimeters and the Union microcalorimeter.
These traditional methods are cumbersome, expensive and difficult to carry out when it is desired to find out the calorific value of a gaseous mixture within an operating plant.
The method of calculating the calorific value described in international patent application WO 99/36767 considers the measurement of two physical properties (the speed of sound and the thermal conductivity). This method was developed using natural gases representative of the whole range of gases encountered in the gas distribution system in Great Britain. The laboratory experiments performed with these natural gases made it possible to determine the speed of sound in these gases and then to correlate these results with the calorific value. As a single property is not sufficient to follow the variations in calorific value according to whether the gases do or do not have a significant content of inert components, a second physical property (the thermal conductivity) was combined with the speed of sound. According to this method the calorific value is deduced by means of the following correlation:CV=a.ThCH+b.ThCL+c.SoS+d.Ta+e.Ta2+f  (2)in which:                CV is the calorific value;        ThCH is the thermal conductivity at temperature TH;        ThCL is the thermal conductivity at temperature TL;        SoS is the speed of sound at ambient temperature;        Ta is the ambient temperature of the gas;        a, b, c, d, e and f are constants.        
The constants were determined by means of a regression obtained on gas samples of different origins in Great Britain. However, this method only uses gases representative of the gases distributed in Great Britain and cannot therefore be generalized.
A second method of determining the calorific value is based on a knowledge of the content of nitrogen and carbon dioxide in the gas, as well as the value of the density of this gas. The proposed relationship was used by Candwell (1967) and is only valid for gases with a Wobbe index of between 43.4 and 44.4 MJ.m−3 (Groningue gas):CV=5.671+61.38d−98.97KCO2−64.57KN2  (3)
where KCO2 is the carbon dioxide fraction and KN2 is the nitrogen fraction in the gas.
However, these last two methods are not applicable to all the gases distributed in Europe.
b) Methods of Measuring the Methane Index
Experimental Determination of the Methane Index
The methane index is generally measured on a CFR/RDH (Cooperative Fuel Research/Removable Dome Head) standard research engine under the operating conditions defined by Christoph et al.: “Evaluation of the antiknock value of gaseous fuels by means of the methane index and their practical application in gas engines” in MTZ 33, April 1972, no. 10.
Chemical Determination of the Methane Index (by Analysis of the Composition)
Another method of determining the methane index was developed by Ryan and Callahan [RYAN et al., Journal of Engineering for Gas Turbines and Power, October 1993, vol. 115/769, and CALLAHAN et al., 18th Annual Fall Technical Conference of the ASME Internal Combustion Engine Division, 1996, ICE-vol. 27–4] and then improved by Waukesha [Selberg, CIMAC Congress 1998], who defined a new index, called the WKI index, similar to the methane index (patent U.S. Pat. No. 6,061,637).
Graphical Determination of the Methane Index
A method of calculating the methane index of a gaseous fuel from its chemical composition was established by Christoph et al. [cf. article cited above]. It consists in bringing the different constituents together into binary or ternary groups for which the methane index is given by the corresponding diagrams. The equation is of the following form:
                    MI        =                              1            100                    ⁢                                    ∑              i                        ⁢                                          y                j                            ⁢                              MI                j                                                                        (        4        )            
where:
MIj is the methane index of the binary or tertiary group j;
yj is the concentration by volume of the mixture j in the total mixture;
MI is the methane index of the total mixture.
This equation can only be used with the ternary diagrams of each mixture group.
Also, certain rules have to be obeyed:                MIj values must not differ by more than 5 points.        At least one group must contain three components.        A group of a single component can be formed provided that the first rule applies.        The very high-knock components (for example butane) must always be included in a ternary group with antiknock components (for example methane).        The C5 and higher components can be added to the butane because they are only present in the gas in trace amounts.        
For mixtures having nitrogen or carbon dioxide contents below 9 and 2% respectively, the index is determined without taking these products into account. The error is less than two index points in such cases.
For higher contents, the methane index is calculated according to the following equation:MI=MI(without)+MI(inerts)−100  (5)
where:
MI(without) is calculated according to equation (4);
MI(inerts) is calculated with the ternary diagram CH4—CO2—N2, in which all the alkanes are classed as methane.
Empirical Determination of the Methane Index
1) Correlation: MI=f(NCV, xCO2, density)
A simple equation based on measurement of the NCV, the density and the carbon dioxide content of the gas was developed by the German company Ruhrgas. This equation, which is based on a reference model (AVL program calculating the methane index from ternary diagrams), is a multiple linear regression of the following form:MI=C1+C2.NCV+C3.ρ+C4.xCO2+C5.NCV.ρ+C6.NCV.xCO2+C7.ρ.xCO2+C8.NCV.ρ.xCO2+C9.NCV2+C10.ρ2+C11.NCV2.ρ+C12.NCV.ρ2+C13.NCV2.ρ.xCO2  (6)
where:
NCV is the net calorific value of the gas;
ρ is the density of the gas;
xCO2 is the carbon dioxide content.
Two further methods were developed from the calorific value or electrical permittivity data of the gas, its density and its carbon dioxide content:                The first method makes it possible to determine the composition of the gases as a function of the NCV, the density and the CO2 content (calculation algorithm), allowing calculation of the methane index with the AVL program based on experimental results on a CFR engine (cf. “Evaluation of the Antiknocking Property of Gaseous Fuels by means of the Methane Number and its Practical Application to Gas Engines. ASME Paper 72-DGP-4, April 1972. Leiker M. et al.”) and calculation of the NCV and the Wobbe index. The nitrogen content of the gas is estimated. The final composition of the gas is determined by an iterative calculation algorithm integrating a series of NCV second-order correlations (EP 0 939 317 A2).        The second method, which is similar to the first, makes it possible to determine the composition of the gas as a function of the electrical permittivity, the density and the carbon dioxide content of the gas. Both the nitrogen content and the NCV of the hydrocarbons are estimated. The final composition of the gas is again determined by an iterative calculation algorithm integrating a series of NCV second-order correlations (EP 1 081 494 A1).2) Infrared Absorption        
Two methods involving infrared absorption which make it possible to follow the variations in methane index are described in WO 98/25128 and WO 00/50874.
Both the empirical methods of determining the methane index afford a precision in the order of ±2 methane index points.
In addition, as these methods utilize variables (for example NCV) whose measurement can be burdensome in material and economic terms, they can only be employed for users who actually measure the NCV, the density and the carbon dioxide content of a gas (as is the case of the German company Ruhrgas, which actually measures these data in its gas distribution stations).
All the methods used to calculate the methane index and the NCV are correlations based on one, two or three physical properties. Because the approach is empirical, a progressive calibration is required to establish any kind of regression.
These techniques have real disadvantages, the first being the need to establish a strong correlation between the energy properties which it is desired to calculate and the physical properties used for this purpose. Moreover, another disadvantage of methods of this kind is that they cannot easily take account of a negative or positive effect of certain components which have a significant influence on e.g. the methane index or the NCV.